vsrc/vsrcask.c, bug #299, add "nop" handler for VSRC_EXTERNAL in SHARED_MODULE

Avoid NAN error when executing "show all"

Thanks to Sergii Baitala, who reported this in
  #299 Shared ngspice: missed VSRC_EXTERNAL handler in VSRCask
  http://sourceforge.net/p/ngspice/bugs/299/

src/spicelib/devices/vsrc/vsrcask.c

@@ -107,6 +107,11 @@ VSRCask(CKTcircuit *ckt, GENinstance *inst, int which, IFvalue *value, IFvalue *
                         -*(ckt->CKTrhsOld + here->VSRCbranch);
             }
             return(OK);
+#ifdef SHARED_MODULE
+        case VSRC_EXTERNAL:
+            /* Don't do anything */
+            return (OK);
+#endif
         default:
             return (E_BADPARM);
     }